Broad area semiconductor lasers have extremely high electrical to optical wall-plug efficiency and low cost. As a result, they are very promising, lightweight, high-power light sources for a variety of applications. However, the poor spectral and beam quality of these lasers allows them to be used only as pumping sources for solid-state or fiber lasers. To improve their spectral and beam quality, research has focused on frequency stabilization and attempting to obtain coherent beam combinations of single mode lasers within laser arrays. To date, two main approaches have been developed: (i) optical injection by an external single mode single frequency laser (a seed laser) and (ii) external cavity stabilization implementing the external grating/mirror that redirects part of the output back to the semiconductor laser “internal” cavity. In principle, both approaches have the potential to obtain a single mode output from the laser array. However, notwithstanding partial achievements, a completely satisfactory solution for the frequency stabilization and coherent combination of individual beams produced by laser arrays has not been obtained. This is due to the inherent limitation of each of the two technologies, as explained below.
The scalability of the external optical injection scheme to the higher power level requires approximately one seed beam of about 25-50 mW for each Watt of output power. In order to create a coherent output beams all seed beams have to be coherent. The technical problems involve splitting the seed beam onto the array and providing the necessary power in the seed beam to produce an output beam of sufficient power.
In order to obtain a coherent output of an entire array in the external cavity design, a sufficiently strong coupling between the lasers in array is needed. To this end, about at least 80% of the emitted radiation has to be redirected back into the laser array. This results in a very low efficiency of the single mode output in the schemes with external cavity.
The technical problem is how to provide a substantial increase in the beam combining efficiency of the phase locked laser array.